Early on March 28, 1979, a combination of electrical and mechanical malfunctions, as well as human error, unleashed dangerous radioactive gases into the environment around the Three Mile Island nuclear power plant in Pennsylvania. It wound up being the worst nuclear disaster in US history.

The Three Mile Island Nuclear Generating Station on March 30, 1979.
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Three Mile Island was owned by utility company Metropolitan Edison at the time of the accident. The facility had two units, one of which is still operational. The other has been shut and sealed since March 28, 1979.

A view of the Three Mile Island nuclear power plant on March 22, 1999.
Reuters

Holly Garnish, who lived in the nearest house to the Three Mile Island plant at the time of the accident, told the Washington Post that it all started with a loud roar that "shook the windows, the whole house."

A sign marks the Three Mile Island nuclear power plant on March 15, 2011.
Jonathan Ernst/Reuters

About half of the reactor core melted from the overheated nuclear fuel, and about 20 tons of radioactive uranium poured out of the reactor core. It covered the steel floor and nearly burned through it.

An aerial view of Three Mile Island on April 2, 1979.
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Other nuclear disasters, like Chernobyl in 1986 and Fukushima in 2011, are considered full meltdowns because the overheating caused the containment structures housing the reactors to split open. Those events released a much larger amount of radioactive material.

An aerial view of the Chernobyl nuclear power plant, the site of the world's worst nuclear accident, in April 1986, days after the explosion.
AP

Fortunately, nobody died because of the Three Mile Island accident. Officials declared a state of emergency, but no official evacuation process had been established for this kind of scenario.

Evacuees in Civil Defense shelter at a local sports arena after the accident at the Three Mile Island reactor on April 2, 1979.
Michael Abramson/The LIFE Images Collection via Getty Images/Getty Images

"It was like something out of a horror movie," Christine Layman, who lived about 7 miles from the plant at the time of the incident, told The Philadelphia Inquirer. "No one knew what was going on."

Gwen Majette, 4, of Middletown, Pennsylvania, sleeps on her mother's lap as Willie Majette reads the morning headlines about the evacuation from the 5-mile area around the Three Mile Island Nuclear Plant on March 31, 1979.
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In total, about 2 million people within a 50-mile radius were exposed to small amounts of radiation because of the Three Mile Island nuclear accident. According to the US Nuclear Regulatory Commission (NRC), the radiation exposure equaled that of a chest X-ray.

The process of cleaning up the reactor took 14 years and cost an estimated $1 billion. The reactor was damaged beyond repair and was sealed shut with concrete following the accident. Its neighboring reactor remained operational.

In an eerie coincidence, a film called "The China Syndrome" about a fictional nuclear power plant disaster came out just 12 days before the Three Mile Island accident. The movie starred Jane Fonda and Michael Douglas.

Jane Fonda and Michael Douglas on the set of "The China Syndrome."
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In the film, one character even says the fictional nuclear accident has the potential to "render an area the size of Pennsylvania uninhabitable" (though the disaster in the film takes place in California).

Actress Jane Fonda and her husband, political activist Tom Hayden, are seen during a press conference outside the Three Mile Island nuclear plant on September 24, 1979.
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But during the Three Mile Island crisis, reactor staff really did fear that the nuclear fuel could melt through the containment structure and seep into the ground (though not through the entire globe).

The Three Mile Island nuclear power plant is seen on March 15, 201.
Jonathan Ernst/Reuters

The nuclear-energy industry, including the NRC, says the radiation in 1979 had no serious health consequences. The accident’s "small radioactive releases had no detectable health effects on plant workers or the public," the NRC website says.

Radiation levels are checked on a group of workers at the Three Mile Island Nuclear Power plant on March 28, 1979.
Rolls Press/Popperfoto via Getty Images/Getty Images

The researchers behind that study relied on the same research method that was used after the Chernobyl nuclear disaster: They studied patients with thyroid cancer to see how many of them had a genetic mutation that makes them more prone to non-radiation-induced thyroid cancer.

A man fishes in the Susquehanna River in front of the Three Mile Island plant on May 30, 2017.
Carlo Allegri/Reuters

Their results suggested that while 83% of thyroid-cancer patients in a control group had this mutation, only 53% of thyroid-cancer patients who'd lived in at-risk locations at the time of the nuclear accident had the mutation.

Because of the new rules, the process of designing and building new nuclear power plants became longer and more costly. Since the Three Mile Island accident, no new nuclear plants have been built in the US.

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